write_riemann: Use riemann-c-client

write_riemann: Use riemann-c-client

Instead of implementing a collectd-specific riemann client, use the
riemann-c-client library to talk to Riemann.

This also adds support for TLS, as an added bonus, and makes it easier to
improve the plugin later on.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>